This invention relates to a method and apparatus to improve production of synthesis gas for manufacturing ammonia. The invention reduces pressure losses in a nitrogen-wash purifier unit.
Processes for manufacturing ammonia from a hydrocarbon and air, via a hydrogen/nitrogen synthesis gas (syngas), are well known. Extraneous syngas components typically include inert gases from the air and/or the hydrocarbon feed, such as argon and methane. When excess air is used in the syngas production, nitrogen is also present in stoichiometric excess, and must be removed from a raw makeup syngas stream or purged from an ammonia synthesis loop to maintain a desired ammonia synthesis reactor feed composition.
U.S. Pat. No. 3,442,613 to Grotz discloses a syngas production method using excess air and cryogenic syngas purification, which relies on a syngas pressure drop upstream of purification for refrigeration. The pressure drop is subsequently made up in a compressor that raises the syngas to ammonia synthesis loop pressure. The method also reduces the rate of recycle or purge gas flow from the ammonia reactor loop due to the upstream removal from the makeup syngas of inerts such as argon and methane in the syngas purification.
U.S. Pat. No. 4,568,530 to Mandelik et al provides a method of ammonia synthesis using high-activity catalyst in the ammonia synthesis reactor. Purge gases are eliminated via a hydrogen enrichment process operating on a sidestream of the syngas recycled to the synthesis loop compressor. The total recycle flow is roughly three times the volumetric flowrate of the makeup syngas.
U.S. Pat. No. 4,681,745 to Pinto recommends using air separation to provide oxygen-enriched air such that reforming produces a synthesis gas with higher hydrocarbon slip than in other ammonia manufacturing systems. A higher concentration of nonreactive gas in the ammonia synthesis is managed by purging from a residual syngas stream following recovery of ammonia product. This approach unloads front-end gas reforming reactors, at the expense of including air separation, but ostensibly enables a smaller purge stream process after ammonia synthesis.
U.S. Pat. No. 5,180,570 to Lee et al describes an integrated process system for synthesizing methanol and ammonia. An ammonia synthesis section uses a nitrogen wash by cryogenic fractionation to purify ammonia syngas, with refrigeration supplied externally and providing no recovery of expansion power in the process.
Gosnell et al, “New Kellogg Brown & Root Ammonia Process,” July 1999, presented at the AlChE Ammonia Safety Symposium, September 1999, describes an ammonia synthesis process utilizing cryogenic syngas purification integrated with an optimized plant front end for syngas generation and a high-activity ammonia catalyst in the synthesis.